1. Field of the Invention.
This invention relates to improved devices for providing hearing protection from exposure to sounds that are intense enough to risk hearing damage or discomfort.
2. Background of the Prior Art
It is well documented that repeated or prolonged exposure to sounds of sufficiently high sound pressure level (SPL) will cause temporary or permanent hearing loss. Earplugs and earmuffs suitable for preventing hearing loss have been widely available, but until recently have all been characterized by a common limitation in that they provide much greater attenuation at high frequencies than at low frequencies and/or provide excessive attenuation at high frequencies. The result was that the user who wanted or needed to hear clearly was prevented from doing so.
More recently, high fidelity earplugs suitable for use with custom earmolds have been introduced by Etymotic Research, Inc., licensed under U.S. Pat. No. 4,807,612 issued Feb. 28, 1989 to Elmer Carlson. These provide a uniform attenuation of approximately 15 dB across the entire audible frequency band, from 20 Hz to 16 kHz, as confirmed by applicants own measurements and, more recently, measurements performed on 16 subjects in the diffuse-field test chamber in the Indianapolis facilities of E-A-R company. These have been well accepted by symphony musicians, rock musicians, and others who need some hearing protection but also need to hear clearly while wearing earplugs.
While giving superb acoustical performance, the Carlson earplug has several limitations:
1. The diaphragm compliance element required by the Carlson earplug is difficult (and thus relatively costly) to manufacture to the tight compliance tolerance required for proper operation.
2. The Carlson earplug requires a precise and relatively large diameter internal sound channel in the accompanying custom earmold in order that the Helmholtz resonance between the acoustic mass intrinsic to that internal sound channel and the compliance of air in the earcanal have the proper frequency. A consequent limitation to the Carlson earplug has been the necessity of obtaining a specially manufactured custom earmold for each ear, where it has been found that the earmold manufacturer must individually measure and "tune" the internal sound channel using a special meter manufactured by Etymotic Research in order to provide the correct value of acoustic mass required for proper operation.
3. As a result of limitations 1 and 2, the Carlson earplug has been deliverable to the end user only at a relatively high overall cost, typically $100 to $125 per pair to the end user. This cost and inconvenience may be of relatively little concern to a professional musician, but they have hampered the widespread application of the Carlson earplug to recreational and industrial users.
The need for an inexpensive improved audibility earplug has become clearer as the ready availability of the Carlson earplug has generated further thinking and discussion. A common problem in industry has been that workers do not wear the mandatory ear protection properly, sometimes complaining that it is difficult to hear properly with standard earplugs in place. Studies reported by Elliott Berger of E-A-R corporation have shown average values of Noise Reduction Rating (NRR) of 2 to 10 dB when NRR is measured in the real-world environment as workers are actually using their earplugs, even on earplugs with rated NRR values of 20 to 30 dB based on laboratory attenuation measurements. Applicants interpret this as an indication that some workers learn how to compromise the performance of hearing protectors (by inserting them only half way, for example) when the attenuation they provide is excessive for the situation. The problem with the half-inserted or partially sealed earplugs is that they still provide excessive attenuation at high frequencies even though the attenuation at low frequencies is reduced, creating a muffled sound that makes it difficult to understand speech or monitor machinery for proper operation. When the workers have a mild to moderate hearing loss, the attenuation of the hearing protector is added to the "internal attenuation" they already endure due to their hearing loss, further increasing their resistance to hearing protection. The most recent estimates are that whereas some two-thirds of factory workers work in environments where the noise level exceeds 85 dB(A) daily weighted average, and thus some hearing protection is needed to prevent hearing loss, three-fourths of those workers need less than 10 dB of protection. In the absence of a readily available improved-audibility earplug, it appears that many workers compromise the performance of their earplugs, or some simply refuse to wear ear protection, risking further hearing loss.
Subsequent to the Carlson earplug, a lower-cost earplug with improved audibility was described in U.S. Pat. No. 4,852,683, issued Aug. 1, 1989 to Mead C. Killion, one of the present applicants. The disclosure thereof is incorporated herein by reference. That patent application describes in greater detail both the prior art and the Carlson earplug. The Killion earplug solved the principal limitation of the Carlson earplug by using damping to render the Helmholtz resonance unimportant, adding an external structure coupled to the sound channel to increase the response characteristics at higher frequencies.
In a preferred "loop-tube" version, the Killion earplug used an external plastic tube formed into a "loop" shape to form a quarter-wave resonance and, by using an appropriately larger diameter in the external loop tube than in the internal sound channel, obtained desireable "horn" action to further improve the high frequency characteristics of the resulting earplug. Good performance has been obtained with this external loop tube, plus an adapter tube containing the damping element, coupled to slow-recovery foam eartips (Reissue U.S. Patent No. 29,487 Reissued Dec. 6, 1977 to Ross Gardner, Jr.) that have internal 0.076" i.d. "#13" vinyl sound tubes. These foam eartips are readily available from Etymotic Research under the designation "ER-3-14 eartips" or from E-A-R corporation under the designation "EARLINKS." Good performance has also been obtained using three-flanged molded eartips available from E-A-R corporation under the designation "UltraFit," when the latter have been specially molded to include an internal 0.076" i.d. sound channel. Applicants' company Etymotic Research has assembled and sold improved audibility earplugs of the latter construction on a limited basis with good results.